This invention relates generally to DC circuit breaker apparatus to be used in a DC circuit such as a DC power transmission system, and more particularly to a type thereof including a device (hereinafter termed commutation circuit) which causes a reverse current to flow through the circuit breaker in a direction reverse to that of a normal current flowing through the circuit breaker apparatus.
FIG. 1 illustrates a DC power transmission system including a conventional DC circuit breaker. In the system, an AC/DC converter 1 converts AC into DC and transmits the DC power to an inverter 3 through a DC circuit breaker apparatus 2 which comprises a circuit interrupter 4 and a commutation capacitor 5 parallelly connected with the interrupter 4.
While the interrupter 4 is being closed, a DC current (hereinafter termed main current) I.sub.0 flow through the circuit breaker apparatus 2 toward the inverter 3.
However, when the interrupter 4 is opened to interrupt the circuit breaker apparatus 2, an arc voltage created between the contacts of the interrupter 4 increases with time, thereby charging the capacitor 5.
By reason of a negative resistance characteristic of the arc, an arc current flowing between the contacts becomes oscillatory with the amplitude increasing with time as shown in FIG. 2. The osillatory current produces a zero point in the main current I.sub.0, and usually the circuit breaker interrupts the main current I.sub.0.
In the above described construction of the circuit breaker apparatus 2, however, the capacitance of the capacitor 5 must be increased in accordance with the rated current of the circuit breaker. Thus, a transmission system of a large current rating requires a capacitor of a large capacitance and hence of an excessively large size.
In order to obviate the above described drawback, another DC circuit breaker as shown in FIG. 3 has been proposed, in which circuit elements corresponding to those shown in FIG. 1 are designated by the same reference numerals.
In the circuit breaker shown in FIG. 3, one terminal of the capacitor 5 is grounded through a resistor 6, while the other terminal is connected to the line of the system downstream of the circuit interrupter 4. While the interrupter 4 is held in closed state, the capacitor 5 is charged from the line voltage of the DC circuit or the DC power transmission system.
When the interrupter 4 opens, an electric arc is created between the contacts of the interrupter 4. The voltage across the separated contacts increases according to the elapse of time, and when the voltage exceeds a predetermined value, a discharge gap 7 connected between the converter side of the interrupter 4 and the grounded terminal of the capacitor 5 conducts, thereby causing a discharge current I.sub.1 oscillating at a frequency determined by the capacitance of the capacitor 5 to flow through the interrupter 4 in a direction reverse to that of the main current I.sub.0.
The discharge current I.sub.1 forms a zero point in the main current I.sub.0 immediately after the opening of the interrupter 4 as shown in FIG. 4, and at the zero point, the interrupter 4 interrupts the main current I.sub.0.
In the above described conventional circuit breaker, the zero point is formed forcibly so as to cause the circuit breaker to positively interrupt current. Furthermore, since the reverse current I.sub.1 providing the zero point is created by discharging the capacitor 5 charged from the line voltage of the transmission system, the capacitance of the capacitor 5 required for this example can be substantially reduced from that of the capacitor used in FIG. 1. In the example shown in FIG. 3, however, the charge of the capacitor is determined by the line voltage of the transmission system. Thus, in a case where the circuit breaker is closed at a time when the line voltage is substantially reduced, and when a fault occurs at this time on the transmission system, the capacitor 5 cannot create reverse current of a sufficient intensity, thereby failing to positively interrupt the main current I.sub.0. In other words, either one of the DC circuit breakers shown in FIGS. 1 and 3 has various difficulties such as insufficient reliability when it is used practically.